With Halley's Help, Sun's Size Is Found to Be Constant

By JAMES GLEICK

Published: February 16, 1988

IT was the era of a peculiar and now-vanishing species, the English gentleman scientist.

''That accurate observer Mr. Stephen Gray'' watched the 1715 solar eclipse from Norton-court. At Badsworth stood ''the Reverend and Learned Mr. Daubuz.'' And ''the curious William Tempest Esq.'' watched from Cranbrook, where he saw the sun ''extinguished but for a moment.''

These eyewitnesses turned out across the southern countryside, pendulum clocks in tow. They were spurred to the task by no less an astronomer than Edmond Halley, of comet fame, who collected their eyewitness accounts of a solar eclipse in the hope of gaining precise information about the long-term trajectory of the moon.

Halley's successors at the Royal Greenwich Observatory have now gone back to his archives to perform an unusual piece of scientific detective work. They contend that a reconstruction of the precise path of the moon's shadow in 1715, which plunged some towns into total darkness and left others in a partial light, settles an international controversy over whether the sun has been shrinking.

They find that it has not; the sun today is the same one that shined on Halley. The honor for the observation made 273 years ago that has proved most crucial to today's argument goes to one Theophilus Shelton of Darrington - ''that curious gentleman,'' as Halley described him. His House Still Stands

From outside his house, which stands in Darrington still, Shelton must have been thrilled by the unique sight of a total eclipse; as he watched, the moon spread across the sun's disk until it blotted out all but a last point of sunlight, suggesting that he was just on the edge of the eclipse's shadow.

''He must have been right on the limit, or near it,'' said Leslie V. Morrison of the Royal Greenwich Observatory. Three French astronomers contended last year that the sun looked significantly larger three centuries ago; Dr. Morrison and his colleagues argue in the journal Nature that the eclipse data show firmly that the sun was virtually the same size as today. Of the many eyewitness accounts from 1715, Shelton's firmly established the northern boundary of the eclipse.

The argument over the sun's diameter has raged for a decade, since John A. Eddy, a solar scientist now at the National Center for Atmospheric Research, got up at a conference and proposed that the sun was shrinking by about one-tenth of 1 percent per century. If the sun shrank, its energy output might be affected, changing the climate on Earth. Many astronomers immediately began trying to prove otherwise.

Irwin I. Shapiro of the Harvard-Smithsonian Institute for Astrophysics, for example, figured that, at such an extreme rate of shrinkage, the sun would vanish to a point in a mere 100,000 years.

''I thought I'd settled that issue a long time ago,'' Dr. Shapiro said. He analyzed observations over the centuries of the transit of the planet Mercury across the solar disk. Many astronomers had noted the exact times of the planet's intersection with the leading and trailing edge of the sun, giving a way of calculating solar diameter.

The results clearly seemed to rule out any large-scale shrinkage. ''I thought it was quite clearly a dead issue,'' he said. Vintage French Data

The French thought otherwise, however, and they resurrected some vintage data from their home institution, the Paris Observatory. Beginning in 1666, Jean Picard, a pioneer in the precise measurement and timing of heavenly bodies, used two large telescopic instruments to observe the sun. He and his successors and assistants diligently wrote down thousands of numbers, which seem to imply a large solar disk, 4 seconds of arc larger than today.

Why should the sun shrink? As the French astronomers saw it, the evidence suggested a ''pulsating phenomenon,'' in which the outer layers of the sun dilated and contracted in cycles over a period of centuries. Not only was the sun larger in the 17th century, they reported, but it also rotated more slowly and showed fewer sunspots.

Judging measurements made by long-dead scientists on ancient sextants and micrometers is a tricky business. The era of the quartz wristwatch was not in sight. There were many opportunities for errors, both random and systematic. Two astronomers at Rice University in Houston recently re-examined the Picard observations and found them wanting.

They raised a variety of down-to-earth questions. Wouldn't the inferior glass of the Paris instruments have smeared the image? Wouldn't the unevenly machined metal screw used to adjust the micrometer lead to significant errors? Couldn't the personal viewing habits of the observers have added a bias?

The British astronomers were not the first to turn to eclipse observations for evidence as to the sun's size. In 1983, Sabatino Sofia, now at Yale University, and several colleagues dug out some detailed observations of a 1925 eclipse over the northeastern United States. Data came not only from observatories, lighthouses and Coast Guard cutters, but from 140 employees of the Affiliated Electric Companies of New York City, who watched the eclipse from rooftops at one-block intervals across Manhattan.